Imaging black holes: past, present and future

Abstract

This paper briefly reviews past, current, and future efforts to image black holes in the radio regime. Black holes seem like mystical objects, but they are an integral part of current astrophysics and are at the center of attempts to unify quantum physics and general relativity. Yet, nobody has ever seen a black hole. What do they look like? Initially, this question seemed more of an academic nature. However, this has changed over the past two decades. Observations and theoretical considerations suggest that the supermassive black hole, Sgr A*, in the center of our Milky Way is surrounded by a compact, foggy emission region radiating at and above 230 GHz. It has been predicted that the event horizon of Sgr A* should cast its shadow onto that emission region, which could be detectable with a global VLBI array of radio telescopes. In contrast to earlier pictures of black holes, that dark feature is not supposed to be due to a hole in the accretion flow, but would represent a true negative image of the event horizon. Currently, the global Event Horizon Telescope consortium is attempting to make such an image. In the future those images could be improved by adding more telescopes to the array, in particular at high sites in Africa. Ultimately, a space array at THz frequencies, the Event Horizon Imager, could produce much more detailed images of black holes. In combination with numerical simulations and precise measurements of the orbits of stars - ideally also of pulsars - these images will allow us to study black holes with unprecedented precision.

arXiv comment10 pages, 3 figures, invited review, http://iopscience.iop.org/article/10.1088/1742-6596/942/1/012001

References in corpus (30)

• The Galactic Center Massive Black Hole and Nuclear Star Cluster
• Viewing the Shadow of the Black Hole at the Galactic Center
• Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre
• The Proper Motion of Sgr A*: II. The Mass of Sgr A*
• High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy
• First Light for GRAVITY: Phase Referencing Optical Interferometry for the Very Large Telescope Interferometer
• The Supermassive Black Hole at the Galactic Center
• An Update on Monitoring Stellar Orbits in the Galactic Center
• A strong magnetic field around the supermassive black hole at the centre of the Galaxy
• Radiative Models of Sgr A* from GRMHD Simulations
• An Unambiguous Detection of Faraday Rotation in Sagittarius A*
• The Black Hole Accretion Code
• General parametrization of axisymmetric black holes in metric theories of gravity
• A Jet-ADAF model for Sgr A*
• Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz
• A size of ~1 AU for the radio source Sgr A* at the centre of the Milky Way
• High Resolution Linear Polarimetric Imaging for the Event Horizon Telescope
• Testing the No-Hair Theorem with Event Horizon Telescope Observations of Sagittarius A*
• Resolved Magnetic-Field Structure and Variability Near the Event Horizon of Sagittarius A*
• Detection of the Intrinsic Size of Sagittarius A* through Closure Amplitude Imaging (includes Online Supporting Material)
• The simultaneous spectrum of Sgr A* from 20cm to 1mm and the nature of the mm-excess
• BlackHoleCam: fundamental physics of the Galactic center
• A Quantitative Test of the No-Hair Theorem with Sgr A* using stars, pulsars, and the Event Horizon Telescope
• Observational appearance of inefficient accretion flows and jets in 3D GRMHD simulations: Application to Sgr~A*
• A General Relativistic Null Hypothesis Test with Event Horizon Telescope Observations of the black-hole shadow in Sgr A*
• Towards the event horizon -- the supermassive black hole in the Galactic Center
• Coupled jet-disk model for Sgr A*: explaining the flat-spectrum radio core with GRMHD simulations of jets
• Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales
• Imaging an Event Horizon: Mitigation of Source Variability of Sagittarius A*
• White Paper on East Asian Vision for mm/submm VLBI: Toward Black Hole Astrophysics down to Angular Resolution of 1~R${}_S$

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• Spectral Energy Distribution of the inner accretion flow around Sgr A* -- Clue for a weak outflow in the innermost region
• Multi-level images around Kerr-Newman black holes
• Numerical black hole initial data and shadows in dynamical Chern-Simons gravity
• The Legacy of Einstein's Eclipse, Gravitational Lensing
• An Illustrated History of Black Hole Imaging : Personal Recollections (1972-2002)
• Strong-field gravitational lensing by black holes
• Exploring higher order images with Fe K_α-lines from relativistic disks: black hole spin determination and bias
• Orbits in a stochastic Schwarzschild geometry
• Images of the accretion disk in Hybrid metric-Palatini gravity
• A multimessenger view of galaxies and quasars from now to mid-century